GEOMORPHOLOGY – FALL 2007
SOIL CLASSIFICATION ASSIGNMENT
NAME __________________________
Materials must be neat, organized, and submitted on time to receive full-credit. (Please staple or
clip papers together)
Part I – U.S. Department of Agriculture (USDA) Soil Classification
of Soil Samples (15 points)
Using the USDA triangle diagram, classify the following hypothetical soil samples based
on the relative percentages of sand, silt, and clay. Note that each component (sand, silt,
or clay) is plotted parallel to the side of the triangle counter-clockwise from its scale, as
shown in the example. See example in Powerpoint presentation

Example: 20% sand, 50% clay, 30% silt.
____________________

Sample 1: 50% sand, 20% clay, 30% silt.
____________________



Sample 2: 10% sand, 45% clay, 45% silt.
____________________
Sample 3: 30% sand, 70% clay.
____________________

Sample 4: 20% gravel, 40% sand, 40% silt.
____________________
Part II – Grain Sized Analysis and Classification of Coarse-grained Soils using the
Unified Soil Classification System (USCS) (40 points)
Refer to the graph showing grain size distribution analyses of Czechoslovakian
sediments. For samples 1, 3 and 4, do the following:

Identify the D10 and D60, which are the particle sizes at which 10 and 60 percent,
respectively, of the sample is finer.

Calculate the uniformity coefficient (Cu), which is Cu = D60 / D10 .

Calculate the coefficient of concavity (Cc ), which is Cc = (D30)2 / [ (D10) (D60) ]
1

State which sample, 1, 3 or 4, is most uniform (i.e., less variation in grain size).
____________________________________________________

State which sample likely has a higher value of porosity and why.
____________________________________________________

Classify the samples according to the USCS (classification charts on following
pages). Assume that the fine-grained fraction in sample 4 is plastic.
Sample 1
Cu _____________
Cc _______________
Poorly graded or Well graded (circle one)
USCS Classification _______________
2
Sample 3
Cu _____________
Cc _______________
Poorly graded or Well graded (circle one)
USCS Classification _______________
Sample 4
Cu _____________
Cc _______________
Poorly graded or Well graded (circle one)
USCS Classification _______________
Note:
No.4 sieve = 4.75mm (commonly used gravel-sand break)
No. 10 sieve = 2.0mm (another commonly used gravel-sand break)
No. 200 sieve = 0.075mm (sand-silt break)
For classifying the above samples per the USCS, use the No. 10 (2mm) as the gravel-sand
break, not the No. 4 (4.75mm) listed in the charts below.
3
SC
C
4
5
Part III –Classifying soils according to NRCS Soil Orders (20 points)
See the general descriptions of the 12 soil orders recognized by NRCS at
http://soils.ag.uidaho.edu/soilorders/index.htm. Use these descriptions together with Chapter 3
of your text to classify the soils shown and described on the following pages.

Order for Soil 1 ________________________________

Order for Soil 2 ________________________________

Order for Soil 3 ________________________________

Order for Soil 4 ________________________________
6
SOIL 1




Color/Texture: brown, silty clay
Relatively unweathered w/ poorly developed horizons
Developed on floodplain, alluvial layering still evident.
Note absence of “B” horizon and buried “A” horizon.
7
SOIL 2




Color/Texture: red, silty clay.
Developed on basalt flow.
Thick, highly weathered and leached.
Well developed “B” horizon w/abundant hematite, limonite and bauxite.
8
SOIL 3




Color/Texture: brown/gray, pebbly and silty clay.
Developed on glaciated landscape
Moderately weathered w/evidence of leaching (“E” horizon) at base of “A” horizon.
Dense clay accumulation in “Bt” horizon.
9
SOIL 4



Color/Texture: brown/gray, sandy silt.
Shallow profile developed on dry, basalt plains and terraces.
Slightly to moderately weathered with secondary mineral accumulation in “Bt” horizon
and hard caliche in “Bk” horizon.
10